Detection processing system

ABSTRACT

A system for scanning an x-ray target in an x-ray imaging system with a charged particle beam. The system for scanning comprises deflecting the charged particle beam to re-illuminate the portion of the object to be imaged in a time period that is sufficiently small to prevent image blurring during image reconstruction. The system for scanning further comprises a scan processing system for processing x-ray transmissiveness information received as a result of a scanning pattern that re-illuminates an object.

BACKGROUND

1. Field of the Invention

The field of the present invention pertains to diagnostic x-ray imagingequipment, including among other things, real-time x-ray imaging methodsand apparatus.

2. Description of Related Art

Real-time x-ray imaging is increasingly being required by medicalprocedures as therapeutic technologies advance. For example, manyelectro-physiologic cardiac procedures, peripheral vascular procedures,PTCA procedures (percutaneous transluminal catheter angioplasty),urological procedures, and orthopedic procedures rely on real-time x-rayimaging. In addition, modem medical procedures often require the use ofinstruments, such as catheters, that are inserted into the human body.These medical procedures often require the ability to discern the exactlocation of instruments that are inserted within the human body, oftenin conjunction with an accurate image of the surrounding body throughthe use of x-ray imaging.

A number of real-time x-ray imaging systems are known. These includefluoroscope-based systems where x-rays are projected into an object tobe x-rayed and shadows caused by relatively x-ray opaque matter withinthe object are displayed on the fluoroscope located on the opposite sideof the object from the x-ray source. Scanning x-ray tubes have beenknown in conjunction with the fluoroscopy art since at least the early1950s. Moon, Amplifying and Intensifying the Fluoroscopic Image by Meansof a Scanning X-ray Tube, Science, Oct. 6, 1950, pp. 389-395.

Another approach to x-ray imaging involves the use of reverse-geometryx-ray imaging systems. In such systems, an x-ray tube is employed inwhich an electron beam is generated and focussed upon a small spot on arelatively large target assembly, emitting x-ray radiation from thatspot. The electron beam is deflected in a scan pattern over the targetassembly. A relatively small x-ray detector is placed at a distance fromthe target assembly of the x-ray tube. The x-ray detector convertsx-rays that strike it into an electrical signal indicative of the amountof x-ray flux detected at the detector. One advantage provided byreverse-geometry systems is that the geometry of such systems allowsx-rays to be projected at an object from multiple angles withoutrequiring physical relocation of the x-ray tube.

When an object is placed between the x-ray tube and the detector, x-raysare attenuated and/or scattered by the object in proportion to the x-raydensity of the object. While the x-ray tube is in scanning mode, thesignal from the detector is inversely proportional to the x-ray densityof the object. The output signal from the detector can be applied to thez-axis (luminance) input of a video monitor. This signal modulates thebrightness of the viewing screen. The x and y inputs to the videomonitor can be derived from the signals that effect deflection of theelectron beam of the x-ray tube. Therefore, the luminance of a point onthe viewing screen is inversely proportional to the absorption of x-rayspassing from the source, through particular areas of the object, to thedetector.

Medical x-ray systems are usually operated at the lowest possible x-rayexposure level at the entrance of the patient that is consistent withimage quality requirements (particularly contrast resolution and spatialresolution requirements for the procedure and the system being used).

Time and area distributions of x-ray flux follow a Poisson distributionand have an associated randomness. The randomness is typically expressedas the standard deviation of the mean flux and equals its square root.The signal-to-noise ratio of an x-ray image under these conditions isequal to the mean flux divided by the square root of the mean flux,i.e., for a mean flux of 100 photons, the noise is +/−10 photons, andthe signal-to-noise ratio is 10.

A relatively high level of x-ray flux makes it easier to yield highresolution images. A high level of x-ray flux can create a potentiallymore accurate image by decreasing the x-ray quantum noise. The x-rayflux should be projected through the object often enough to allow aframe rate (the number of times per second that an object is scanned andthe image refreshed) which produces an acceptable image picture andrefresh rate at a video display device.

In a reverse-geometry medical imaging system, the desire for high levelsof x-ray flux normally requires extended bombardment of an x-ray tubetarget assembly by a high energy electron beam. In creating x-rays inresponse to an electron beam, the x-ray target assembly is raised tohigh temperatures; in some systems, the target assembly material isheated to temperatures in excess of 1000 degrees centigrade. Prolongedexposure of the target assembly to high temperatures can cause meltingor cracking of the target assembly material due to thermal stress. Evenif the prolonged exposure does not immediately cause the target assemblymaterial to fail, such exposure can cause long-term damage that affectsthe longevity of the target assembly.

Thus, several conflicting factors, among them image resolution, framerate, and the thermal qualities of x-ray target assembly materials, maywork to limit the usefulness of conventional x-ray imaging systems.Maintaining an electron beam bombardment of an x-ray tube targetassembly for a sufficient period of time to satisfy flux/frame raterequirements may result in damage to the target assembly material.However, reducing the flux/frame rate requirement to prevent damage tothe target assembly may result in diminished image quality.

Therefore, there is a need for an x-ray imaging method and system thatis capable of addressing the shortcomings of the prior approaches. Thereis a need for a method and system that can provide a high level of x-rayflux to each portion of an object being imaged during a short period oftime while preventing damage to the target material and increasingtarget longevity.

SUMMARY OF THE INVENTIONS

The present invention comprises a system and method for scanning acharged particle beam across a target assembly in an x-ray source toemit x-ray beams to generate an image of an object. An aspect of theinvention comprises moving an electron beam over certain positions onthe target assembly more than once per image frame in order to increasethe flux provided to each portion of the object to be imaged, while atthe same time increasing target longevity.

Another aspect of the invention comprises a scanning system forprocessing x-ray transmissiveness information that results fromrescanning portions of a target assembly. According to one feature ofthis aspect, the scanning system comprises circuitry to combine two ormore pieces of x-ray transmissiveness information, that relate to thesame relative detector xray source locations, into a single combinedpiece of x-ray transmissiveness information.

These and other objects and aspects of the present inventions aretaught, depicted and described in the drawings and the description ofthe invention contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing components of one embodiment of anx-ray imaging system according to the present inventions.

FIG. 2 is an enlarged cross-sectional view of a portion of a collimationgrid and target assembly for use in an imaging system.

FIG. 3 is a front view of an embodiment of a collimation grid.

FIG. 4 is a diagram of x-ray paths emanating from a row of aperturesthat pass through an object and cover the multi-detector array.

FIG. 5 is a diagram of a stepping pattern of an electron beam across atarget assembly.

FIG. 6 is a diagram of an alternate stepping pattern of an electronbeam.

FIG. 7 is a diagram of an alternate stepping pattern of an electronbeam.

FIG. 8 is a block diagram showing the components of an embodiment of animage reconstruction system.

FIG. 9 is a diagram of an embodiment of a detection module.

FIG. 10 is a diagram of a scan processing system according to anembodiment of the invention.

FIG. 11 is a diagram of an interface module according to an embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

System Overview

FIG. 1 is a diagram showing the high level components of an embodimentof a x-ray imaging system according to the invention. X-ray source 10includes an electron beam source comprising a power supply which canoperate x-ray source 10 at about −70 kV to −120 kV. In the presentembodiment, this voltage level produces a spectrum of x-rays ranging to120 keV. Electron beam 40, which is generated within x-ray source 10 bya charged particle gun, is deflected over the surface of a targetassembly 50 (which is a grounded anode in an embodiment of theinvention) in a predetermined pattern, e.g., a scanning or steppingpattern. X-ray source 10 includes a mechanism to control the movement ofelectron beam 40 across target assembly 50, such as a deflection yoke 20under the control of an electron beam pattern generator 30.

A preferred x-ray source 10 is disclosed in copending U.S. patentapplication Ser. Nos. 09/167,329 and 09/167,524, filed concurrently withthe present application, both of which are incorporated by reference intheir entirety. A method and apparatus for generating and movingelectron beam 40 across target assembly 50 is disclosed in commonlyowned U.S. Pat. No. 5,644,612 which is incorporated herein by referencein its entirety.

In FIG. 1, a collimating assembly is located between target assembly 50of x-ray source 10 and a multi-detector array 60. In the preferredembodiment, the collimating assembly is located between target assembly50 and the object 100 for which an image is to be obtained. Thepresently preferred collimating assembly is collimator grid 70,containing a plurality of x-ray transmissive apertures 80 arranged in agrid pattern. Collimator grid 70 is designed to permit passage of x-raysforming a diverging beam 135 that directly intercepts multi-detectorarray 60. In an embodiment, collimator grid 70 utilizes a coolingassembly and beam hardening filters. Examples of preferred collimatorgrids and beam hardening filters that can be utilized in the inventioninclude these depicted and disclosed in U.S. Pat. No. 5,610,967 andcopending U.S. patent application Ser. No. 09/167,639 filed concurrentlywith the present application, both of which are hereby incorporated byreference in their entirety.

In operation, electron beam 40 preferably dwells at location 110 ontarget assembly 50 which is located substantially at a position wherethe axis 90 for a particular aperture 120 of collimator grid 70intersects the target assembly 50. As the electron beam 40 strikestarget assembly 50 at location 110, a cascade of x-rays 130 is emitted.Only the portion of the cascade of x-rays 130 whose path liessubstantially along axis 90 pass through aperture 120 and form adiverging x-ray beam 135. The shape of x-ray beam 135 is influenced bythe shape of aperture 120. For instance, if the aperture is square thex-ray beam 135 takes on a generally truncated pyramidal shape. If theaperture is circular, x-ray beam 135 takes on a generally conical shape.In a preferred embodiment, the shape and area of the aperture is suchthat the area of maximum divergence of the x-ray beam 135 issubstantially the same as the dimensions of the x-ray capture surfacefor multi-detector array 60.

Multi-detector array 60 comprises a plurality of discrete detectors(referred to herein as “detector elements”) 61 arranged in an array.Each detector element 61 includes a x-ray surface having a capture areafor detecting x-rays. Each detector element is capable of independentlymeasuring the amount of x-rays that strike it. When an object 100 isinterposed between the x-ray source 10 and the multi-detector array 60,some of the x-rays in x-ray beam 135 will pass through a portion ofobject 100, and if not scattered or absorbed, will strike the detectorelements that make up multi-detector array 60. The x-rays that strikeany individual detector element comprise a portion of x-ray beam 135that is referred to herein as an x-ray beam subpath.

In a preferred embodiment, each detector element comprises componentsfor measuring the quantity of x-ray photons that strike the detectorelement and outputting a signal representative of that measurement.Alternatively, each detector element includes components for generatingan electrical signal generally proportional to the total energy of thex-rays that strike the detector element. The magnitude of the generatedelectrical signals corresponds to the flux intensity of the x-rays fromthe detected x-ray beam subpath of x-ray beam 135. Utilizing amulti-detector array 60 that independently measures the x-rays whichstrike each detector element results in the generation of x-raytransmissiveness information that is proportional to the x-ray fluxpassing through object 100 along particular x-ray beam subpaths. Theresulting intensity data can be used or manipulated to create arepresentation of object 100, i.e. a representation of the x-raytransmissiveness of object 100, which can be displayed on monitor 140.

X-ray transmissiveness information obtained from the detector elements61 pertinent to specific image pixels are reconstructed by imagereconstruction system 65. In an embodiment, image reconstruction system65 also performs control functions and display preparation for the x-rayimaging system. Operational instructions and control of the x-ray source10, detector 60 and image reconstruction system 65 are made through acontrol workstation 150. Control workstation 150 also receivesoperational and status information from the various components of thex-ray imaging system.

FIG. 8 depicts a block diagram of an embodiment of image reconstructionsystem 65. The image reconstruction system 65 comprises a PCI interface1010 which connects to control workstation 150. In an embodiment, adetection module 700 comprises the components of multi-detector array 60and receives x-ray transmissiveness information. Alternatively,multi-detector array 60 is physically separate from the imagereconstruction system 65 and the detection module 700 comprisescomponents to receive data signals from the multi-detector array 60.Image reconstruction chassis 1005 comprises an interface module 710, oneor more plane reconstruction modules 730, an image selection module 750and an image preprocessor 760. The various components on the imagereconstruction chassis 1005 are interconnected via one or more busses1100, which also include control lines. Video post processor 770 iscoupled to display monitors 1080. An image construction system andmethod is disclosed in copending U.S. patent application Ser. No.09/167,413 and application Ser. No. 09/167,171, filed concurrentlyherewith, which are hereby incorporated by reference in their entirety.

Referring to FIG. 2, shown is an embodiment of a target assembly 50 thatcan be used in the present invention. Target assembly 50 comprises anx-ray generating layer 230 which is supported by a support layer 210. Athermal buffer 225 can be disposed between the x-ray generating layer230 and the support layer 210. The preferred target assembly isdisclosed in more detail in U.S. patent application Ser. No. 09/167,413and application Ser. No. 09/167,171 filed concurrently with the presentapplication, which is hereby incorporated by reference in its entirety.

In FIG. 2, a cooling chamber 240 is located between the target support210 and collimation grid 70. A cooling fluid can be allowed to flowwithin cooling chamber 240 to cool the target assembly 50. Collimationgrid 70 comprises a plurality of x-ray transmissive apertures 290.

According to an embodiment, the central axis 90 of each of the apertures290, is aligned with center of the multi-detector array 60 (FIG. 1). Inother words, the axes of apertures within the collimation grid 70 arenot parallel to each other, but form an acute angle to a lineperpendicular to the output face 270 of the collimation grid 70. Forexample, an embodiment of a collimation grid for a chest x-rayapplication comprises apertures forming an angle with a lineperpendicular to the output face 270 of the collimation grid 70 ofbetween 0° at the center of the collimation grid 70 to as much as 20° atthe edge of the grid 70. A mammogram application on the other hand mayhave a collimation grid 70 comprising apertures forming an angle with aline perpendicular to the output face 250 ranging to 45° at the edge ofthe grid. Thus, a preferred scanning beam x-ray imaging system allowsdifferent collimation grids to be selected and used depending on theparticular medical application.

X-ray absorbent portion 280 of collimation grid 70 is designed to absorberrant x-rays so that they do not irradiate the object. This isaccomplished by fabricating the preferred collimation grid 70 withsufficient thickness so that the x-ray radiation passing through anaperture 290 towards the multi-detector array 60 is substantiallygreater than the cumulative x-ray radiation passing through x-rayabsorbent portion 280 in all directions other than toward themulti-detector array 60. Such errant x-rays would provide the patientand attending staff with x-ray dosage while contributing no meaningfulinformation to the image.

Referring to FIG. 3, collimation grid 70 is preferably of a shaperesembling an octagon, with circular shaped portions 330 and straightline edges 320, although any shape that can be used in an x-rayscanning-beam system can be employed with the present invention. X-raytransmissive apertures 300 are arranged into a pattern of rows andcolumns. In an embodiment, the number of rows and columns is the same,so that the arrangement of x-ray apertures 300 is in the pattern of asquare. Further, the pitch 340 between apertures 330 is the same for allapertures in both the horizontal and vertical directions. The dimensionsof collimation grid are preferably ten (10) inches in diameter betweencircled shape portions 330 and a distance of nine (9) inches between thecenters of straight line edges 320 and the preferred pitch 340 isbetween two (2.0) millimeters and two point five (2.5) millimeters. Thesize and shape of the apertures collimation grid that can be used withthe present inventions are dependent on the material used inmanufacturing the collimation grid as well as the particular applicationto which the invention are directed. The above described shape and sizeare not intended to be limiting in any way.

Scanning Method

To form an image for display, the electron beam 40 is stepped across thex-ray target assembly at positions opposing a group of apertures in thecollimation grid that form a particular area of the collimation grid 70.X-ray transmissiveness information is measured at the multi-detectorarray 60 for x-rays emanating through each of the apertures. Themeasured x-ray transmissiveness information is mathematically combinedto generate image data.

In an embodiment of an imaging system for medical applications, at least15 and preferably 30 frames per second should be produced. A frame is acomplete image, where all apertures that are required to emit x-rays doemit x-rays that provide information to the multi-detector array, fromwhich an image is reconstructed. For a preferred embodiment using a 100by 100 aperture collimation grid, the time for completing a scan of allof the positions on the target assembly for a single frame ranges fromapproximately 66.7 milliseconds (15 frames per second) to approximately33.3 milliseconds (30 frames per second). Although the total frame timecan range from 33 to 67 milliseconds, any object in the field of view ofthe x-ray source 10 is preferably illuminated for less than 12milliseconds to minimize motion blurring that can result from motionwithin the object. When the x-ray imaging system is being utilized forcardiac imaging applications, objects within the field of view of x-raysource 10 are preferably illuminated for less that 4 milliseconds.

During operation, the electron beam is dwelled at a position on thetarget assembly for a fixed time period, which is referred to as a dwelltime (T_(Dwell)), and is then deflected from the current dwellingposition to a next dwelling position on the target assembly. Dwellingthe electron beam at a position and then deflecting the electron beam toa next position is referred to as a step. During each dwell time, onex-ray path from an aperture passes through and illuminates a portion ofthe object and provides x-ray transmissiveness information about thatportion of the object to the multi-detector array.

Referring to FIG. 4, an object 401 containing a smaller object or regionof interest 400 is interposed between collimation grid 260 andmulti-detector array 110. A first x-ray path 420 from aperture 410 in arow of apertures includes x-rays that travel through object of interest400. A second x-ray path 440 from aperture 430 also includes x-rays thattravel through object of interest 400. It can be seen that there are anumber of apertures 415 between first aperture 410 and second aperture430. A x-ray path from each of the apertures 415 includes x-rays thattravel through object of interest 400 and provide information aboutobject of interest 400 to the multi-detector array 110. However, a x-raypath 470 from aperture 460 does not include any x-rays that pass throughobject of interest 400. This is also the case for an x-ray path fromaperture 480. X-rays emitted from apertures 460 and 480 will provide nomeaningful information about object of interest 400 to themulti-detector array 110.

For any set of apertures used to illuminate an object of interest 400,the time between a dwell at the first aperture that illuminates theobject of interest to a dwell at the last aperture is referred to as theillumination time (T_(illumination)). For the example depicted in FIG.4, the time between a dwell at aperture 410 and at aperture 430, whilealso dwelling at each of the apertures 415 in between, is theillumination time (T_(illumination)) of the object of interest 400 whichis in the field of view of an arrangement of multi-detector array 110and collimation grid 260. As previously stated, the preferredillumination time for an object of interest in the field of view of thex-ray imaging system is less than 12 milliseconds and more preferablyless that 4 milliseconds when the x-ray imaging system is being utilizedfor cardiac imaging applications.

An increased T_(illuminatin) is achieved by stepping across an area ofthe target assembly corresponding to particular apertures more than oncein creating each frame. The illumination of portions of an object morethan once in a frame may be done for a number of purposes such asproviding greater flux to the object for a clearer image whilepreventing target overheating of the target assembly materials. Inrestepping any portion of the target assembly, any part of the objectthat is imaged within a single image frame should not include normalmotion that takes place within the body.

For instance, if a beating heart is being imaged, a second step at thesame aperture that takes place too long after a first step will includeimaging information with the heart in different positions. On a displaythe image of the heart will appear blurred. To prevent motion from beingincluded in a single image frame, any portion of the object being imagedshould be re-illuminated within a short enough time span to limit theintroduction of motion into an image created within a single frame.

It is also preferred for maintenance and longevity of the target thatthe time between a first dwell time and a second dwell time at anyparticular position on the target assembly allows time for the target tosufficiently cool down after the first dwell time at that position onthe target assembly. By creating the total desired flux over two x-rayemitting sessions separated by a period of time, the target is given anopportunity to cool down from the effects of the electron beambombardment from the first dwell time prior to further electron beambombardment from a second dwell time. In an embodiment, the periodbetween the first dwell time and second dwell time is greater than 100microseconds and is preferably at or greater than 300 microseconds.Alternate embodiments comprise the rescanning of particular portions ofthe target any number of times to generate the desired flux, including 3or more scans through the same aperture in a single frame

In an embodiment of an imaging system utilizing a tungsten-rheniumtarget assembly material, the dwell time at each position isapproximately 1 microsecond, while the time for deflecting or sweepingthe electron beam from one position to the next is approximately 280nanoseconds. The dwell time should be limited to prevent excessivetemperature rise in the target material which could damage the targetassembly. Some factors used in determining the maximum dwell time(T_(Dwell)) are the material composition of the target assembly,including the melting point, specific heat and thermal conductivity ofthe target assembly materials, the electron beam power, and the size ofthe dwell spot on the target assembly.

Referring to FIG. 5, a pattern that can be used for stepping electronbeam 40 across a target assembly 50 is a serpentine pattern or reversed“S” pattern. The serpentine pattern is generated by stepping theelectron beam across first row 510 from a position on the targetassembly opposing the left most aperture of the collimation grid to aposition opposing the right most aperture, while dwelling at a positionopposing each aperture in that row of the collimation grid. Afterdwelling at all of the desired dwelling positions in the first row 510,the electron beam is deflected to second row 530, adjacent to first row510 along path 520. In stepping across second row 530 the electron beamdwells on the target assembly opposite the right most aperture and thencontinues for all desired positions opposing apertures in the row toleft most aperture. After dwelling at all of the desired dwellingposition in the second row 530, the electron beam is deflected to thirdrow 550 along path 540. Third row 550 is stepped across in the samemanner as first row 510, i.e., from left to right. This pattern iscontinued for the remainder of the target assembly. After steppingthrough each desired dwelling position, the data for a single imageframe is completed. By stepping at desired dwelling positions oppositeapertures in a preferred 100 by 100 collimation grid, an image of anarea of an object can be constructed.

An embodiment of a stepping pattern where each position in a row isrestepped is shown in FIG. 6. The electron beam is stepped along firstrow 600 in the same manner as in the reverse “S” pattern above. Theelectron beam is then preferably deflected over path 610, from aposition opposing the right most aperture of first row 600 to a positionopposing the left most aperture of the second row 620. After steppingacross second row 620, the electron beam is preferably deflected, asshown by path 630, back to the left most position in first row 600,thereby allowing first row 600 to be stepped across a second time.Second row 620 is then stepped across again in the same manner asdescribed above. The reverse “S” two row restepping pattern continuesfor each pair of desired rows of the target assembly, such that eachdesired pair of rows is stepped across twice before moving on to thenext set of rows. Stepping across a single row in a preferred embodimenttakes approximately 150 microseconds.

It may also be desired that the field of view to be imaged is less thanthe maximum field of view created by dwelling at dwelling positionsbehind the outer perimeter apertures in a collimation grid (which is aten inch diameter for the preferred collimation grid). A smaller fieldof view can be created by scanning a smaller area of the target assembly(e.g., in a central area of the collimation grid that is smaller thanthe area of the entire grid). This can be accomplished, for example, bydwelling at dwelling positions within the area defined by the central 71by 71 apertures of a collimation grid that has 100 by 100 apertures. Inthis arrangement the beam is preferably stepped across three adjacentrows twice prior to moving on to an adjacent set of three rows in areverse “S” three row restepping pattern. One benefit of this scanningpattern is that it allows sufficient cool down time for each position onthe target.

As shown in FIG. 7, row 760 is stepped across left to right startingfrom a dwelling position on the target opposing the left most apertureto a dwelling position on the target opposing the right most aperture.Row 770, and then 780, are thereafter stepped across in the same manneras row 760. After stepping across row 780, row 760 is restepped in thesame manner it was originally stepped, followed by resteps across rows770 and 780. Rows 790, 800 and 810 are then stepped across in the samemanner as rows 760, 770 and 780. The pattern continues for all desiredrows of the target opposite each row of apertures in the collimationgrid.

For each of the above described stepping patterns, the preferredcollimation grid comprised a square array of 100 by 100 apertures. Thenumber of rows stepped prior to restepping the same number of rows is afunction of balancing the cool down time T_(Cool Down), which isdependent on the target material, and the time required to excludemotion in each portion of the image, which is dependent on theparticular portion of the anatomy being imaged.

It is also possible to use the above described method to create a minifield of view. This is accomplished by stepping across an area on thetarget assembly the size of which corresponds to even fewer apertures inthe collimation grid. For example, a mini field of view can be createdby stepping across an area of the target assembly corresponding to aregion of 50 by 50 apertures located in the central area of thepreferred collimation grid. In this mini field of view, four rows arepreferably stepped across once and then a second time, before going onto the next set of four rows creating a reverse “S” four row resteppingpattern. Alternatively, the area to be scanned in the smaller and minifield of view image areas need not be in the central area of thecollimation grid, nor does the area to be scanned necessarily need to bea square or rectangular shape.

An important factor in selecting a scanning pattern is that the timebetween dwells at a single position on the target assembly for an objectin the field of view is selected such that all the positions on thetarget assembly that illuminate the object do so within a sufficienttime period to eliminate blurring. In an embodiment, this time betweendwells at a single position on the target assembly is also at least 100microseconds and preferably 300 microseconds to prevent overheating atthe target assembly. In practice, the smaller the field of view, themore rows that need to be stepped once prior to restepping any positionwithin those rows. This is to maintain the time between steps of a sameposition in a row at a time period sufficiently long enough to preventdamage to the target.

Scan Processing System

The scan processing system of the present invention comprises componentsto efficiently process and receive x-ray transmissiveness informationwhen portions of the object under examination are re-illuminated aplurality of times during a single frame. According to an embodiment ofthe invention, the scan processing system is a component of thedetection module 700 shown in FIG. 8.

Referring back to FIG. 8, shown is a block diagram of an embodiment ofimage reconstruction system 65. Image reconstruction system 65preferably comprises an image reconstruction chassis 1005, whichcomprises the components to reconstruct a display image from x-raytransmissiveness information received at the detection module 700.Components within an embodiment of image reconstruction chassis 1005include an interface module 710 to receive x-ray transmissivenessinformation from detection module 700, one or more plane reconstructionmodules 730 to reconstruct image pixels, an image selection module 750to select image pixels for display, and an image preprocessor 760 toprocess image data for display.

According to an embodiment, detection module 700 is a component of theimage reconstruction system 65. Alternatively, detection module 700 isphysically and functionally separate from the image reconstructionsystem 65, and therefore detection module comprises components to sendand receive data signals from the imaging chassis of imagereconstruction system 65. In the embodiment shown in FIG. 8, detectionmodule 700 comprises the components of multi-detector array 60 to detectx-ray transmissiveness information that passes through the object underexamination. Alternatively, multi-detector array 60 is physicallyseparate from the image reconstruction system 65 and the detectionmodule 700 comprises components to receive and process data signals fromthe multi-detector array 60.

In operation, each time the electron beam dwells at a position on thetarget assembly associated with a particular aperture in the collimator,a complete set of data points, one for each detector element, isgenerated. For example, if the multi-detector array 60 comprises anarray of 48×48 detector elements, and if the electron beam is dwelled ata particular location on the target for about 1.04 microseconds, withabout 0.24 microseconds to move the electron beam to the next position,then the system will generate 2,304 data points approximately every 1.28microseconds. This averages out to a rate of approximately 2gigasamples/second. Using a collimator grid having an array of 100×100apertures, the total amount of discrete pieces of data that is processedfor a single frame, which includes the use of x-ray paths from each ofthe collimator apertures only once, is approximately 23,040,000 datasamples.

To generate an accurate image, the read-modify-write cycle for eachcombination of new x-ray transmissiveness information should occur fastenough so that the information can be processed before a new set of datasamples arrives. In the presently preferred embodiment, the data samplesare produced at a rate that will allow a refresh rate compatible withstandard video display devices. Thus, the scan processing system shouldhave the capability to process the incoming data samples at a fastenough rate to be compatible with a video display device, as well asbeing fast enough to process data samples before a new set of datasamples arrived from the next aperture.

When particular target assembly locations are re-scanned within a singleimage frame as described in the prior section, the number of datasamples to be processed per frame increases as a multiple of the numberof re-scans to perform. To keep from falling behind in the generation ofimage pixels, an embodiment of the invention includes a scan processingsystem to dynamically combine the pieces of x-ray transmissivenessinformation that pertain to the same collimator aperture/detectorelement pairing. The resulting x-ray transmissiveness information isthen transmitted to the interface module 710 of imaging chassis 1005 forfurther processing, including image reconstruction processing.

FIG. 9 diagrams an embodiment of a detection module 700. The detectionmodule 700 includes a multi-detector array 60 to receive x-rays thatpass through the object to be imaged. As stated above, the preferredmulti-detector array 60 comprises a 48 by 48 array of detector elements.Each detector element produces a signal representative of the quantityand/or magnitude of x-ray photons that strike it during a particularmeasurement period. In an embodiment, the x-ray transmissivenessinformation produced by each detector element is a three bit digitalsignal. The preferred multi-detector array is described in more detailin copending U.S. application Ser. No. 09/167,318, filed concurrentlyherewith, which is hereby incorporated by reference in its entirety.

The x-ray transmissiveness information produced by each detector elementis transmitted to a scan processing system 701. According to anembodiment, scan processing system 701 comprises a scan processingmodule 703, a sum cache memory 702, and a FIFO 704. Scan processingmodule 703 comprises circuitry and/or hardware/software to sort andprocess the incoming data samples from the multi-detector array 60. Onetype of operation performed by scan processing module 703 include a datasumming operation to sum x-ray transmissiveness information obtainedfrom the same detector element for emissions from a same collimatoraperture when the stepping pattern dwells the electron beam at a sameposition on the target assembly more than once to during a single frame.Sum cache memory 702 stored partially summed x-ray transmissivenessinformation during the summing operations. The completed x-raytransmissiveness information is output from the scan processing module703 to a FIFO 704. FIFO 704 regulates the transfer of data from scanprocessing system 701 to a Data Mux/Transmitter 706. DataMux/Transmitter 706 performs the operation of sending data to theinterface module 710 of interference chassis 1005 (FIG. 8). Controlsignals for operating scan processing system 701 and multi-detectorarray 60 are received from interface module 710 at a De-Mux/Receiver708.

According to an embodiment, detection module 700 can operate in a numberof modes which affect whether and how the detection module 700 combinesthe x-ray transmissiveness information obtained from the multi-detectorarray 60. The choice of an operating mode also affects when the x-raytransmissiveness information is output from the detection module 700 tointerface module 710.

For example, the operating modes includes an “image acquisition” mode,in which x-ray transmissiveness information from multiple x-rayilluminations within a frame which correspond to the same collimatoraperture/detector element pairing are combined by the scan processingmodule 703. In an embodiment, x-ray transmissiveness informationcomprising 3-bit data from each detector element is received from themulti-detector array 60. If portions of an object are re-illuminatedduring a single frame, then related items of 3-bit data are summed bythe scan processing module 703 to generate 4 bit x-ray transmissivenessinformation used to for image reconstruction.

A second mode is a “sensor” mode in which 3-bit data from themulti-detector array 60 is directly transmitted to the imaging chassis1005. The sensor mode can be used as a diagnostic mode, in which onlyspecific collimator apertures are illuminated upon a specific timingcontrol signal.

A third mode is an “image test” mode. In the image test mode, x-raytransmissiveness information is combined in the same way as in the imageacquisition mode. However, the summed x-ray transmissiveness informationis output to the imaging chassis 1005 only after a timing control signalassociated with a specific scanned collimator aperture is received.

Another mode is an “alignment” mode, which is used to align themulti-detector array 60 with the apertures of the collimator grid 70. Inthe alignment mode, when 3-bit values are generated by the detectorelements of an 48 by 48 multi-detector array 60, 48 nine-bit values aretransmitted for each collimator aperture. Each of these nine-bit valuesrepresent the sum of 48 detector elements for a single illuminationthrough a collimator aperture. The 48 nine-bit values are further summedto provide a 15-bit value representing the total intensity received atall the detector elements for a single illumination of a collimatoraperture. According to a method of performing alignment, successiveilluminations can be performed against particular collimator apertureswhen adjusting the relative positioning between the collimator grid 70and multi-detector array 60. The maximum intensity value measured by themulti-detector array 60 in alignment mode identifies an optimalalignment position of the components of the imaging system.

Referring to FIG. 10, shown are components of a scan processing system701 according to an embodiment of the invention. In the presentembodiment, the circuitry of FIG. 10 processes data received from asingle row of detector elements. Six such circuits can be organized inparallel to receive and process data from six rows of detector elements.If the imaging system employs a 48 by 48 array of detector elements,then eight such groups of parallel circuitry can be employed to receiveand process data from all 48 rows of detector elements.

In FIG. 10, 3-bit x-ray transmissiveness information from a single rowof detector elements is received at an input register 819. The inputregister 819 provides the x-ray transmissiveness information to acounter 820.

If the detection module 700 is in image acquisition mode, the x-raytransmissiveness information is sent from counter 820 to an adder 830.At adder 830, the current x-ray transmissiveness information is added toprior x-ray transmissiveness information that was generated as a resultof one or more prior x-ray emissions from the same collimator aperturein the same frame. The resulting output from adder 830 is provided tomultiplexor 850, which then provides the summed x-ray transmissivenessinformation (through output registers 871 or 881) to either sum cachememory 702 or FIFO 704. If there are no more re-scans to be performed inthe frame for the collimator aperture corresponding to that summed x-raytransmissiveness information, then the summed x-ray transmissivenessinformation is sent to the FIFO 704 for transmission to the interfacemodule 710 of imaging chassis 1005 (FIG. 8). If additional re-scans areto be performed, then the summed x-ray transmissiveness information issent to the sum cache memory 702.

The sum cache memory 702 is used to temporarily store partially summeddata from multiple passes of scan lines in a single frame. The sum cachememory should be large enough to contain the partially summedinformation for the number of rows of collimator apertures beingprocessed. For example, if the scanning pattern is configured such thatthere can be up to 16 rows of collimator apertures are partially scannedat any given time, then the sum cache memory 702 should be capable ofsupporting up to 16 rows of partially summed data. In an embodiment, thesum cache memory 702 comprises memory components that can hold up to 16lines×128 collimator apertures of data.

According to an embodiment, sum cache memory 702 comprises a left cachememory unit 920 and a right cache memory unit 930. Partially summed datacan be stored in either the left 920 or right 930 cache memory units. Inoperation, partially summed data is read from one cache memory unit, iscombined with additional data at the adder 830, and if the newly summeddata is not yet complete, the newly summed data is thereafter stored atthe other cache memory unit. The partially summed data from a priorcollimator aperture in the same frame is sent to adder 830 throughmultiplexor 860, from either left cache memory unit 920 or right cachememory unit 930, through left input register 870 or right input register880 respectively. Although the disclosed sum cache memory componentsutilize the terms “right” and “left”, that terminology is merely forpurposes of explanation and should not be construed as limiting in anyway.

If the detection module is in alignment mode, then counter 820 sendsx-ray transmissiveness information to adder 840, where it is added toprior values received from other detector elements in the same row ofdetector elements. According to an embodiment utilizing a multi-detectorarray having a 48 by 48 array of detector elements, the x-raytransmissiveness information for each row is summed into four groups. Afirst group for columns 1-12, a second group for columns 13-24, a fourthgroup for columns 25-36, and a fourth group for columns 37-48. Then, thedata from groups of four rows are summed. For example, Row 1 data issummed with data from Row 7, Row 13, and Row 19. The combinedinformation is then output in twelve cycles with the output during eachcycle preferably being one set of data for twelve columns of detectorelements.

FIFO 704, which preferably comprises sub-FIFO memories 871 and 872,stores x-ray transmissiveness information and outputs that informationto Data Mux/Transmitter 706 (FIG. 9). Data Mux/Transmitter 706multiplexes the x-ray transmissiveness information and then transmitsthat information to the interface module 710 of imaging chassis 1005(FIG. 8).

FIG. 11 diagrams an embodiment of an interface module 710. Signalsrepresenting x-ray transmissiveness information sent from detectionmodule 700 to interface module 710 are received and demultiplexed bydata receiver 718. The x-ray transmissiveness information received bydata receiver 718 is serially fed into FIFO memory 720. FIFO memory 720regulates the transfer of data that is sent to sensor data processor722. If the imaging system is in image acquisition mode, then signalsrepresenting x-ray transmissiveness information is passed from sensordata processor 722 to the image pixel reconstruction modules of theimage reconstruction system 65. If the imaging system is in sensor oralignment modes, then sensor data processor 722 sends x-raytransmissiveness information to a sensor and alignment memory 714.Sensor and alignment memory 714 stores information for transmission tocontrol workstation 150. Line pointer memory 716 stores controlinformation received from control workstation 150 that represents theorder and stepping pattern of the electron beam and provides signalsrepresenting this information to detection module 700. Interfacecontroller 712 receives and transmits signals between the interfacemodule 710 and control workstation 150, and also transmits signals tothe detection module 700.

While the embodiments, applications and advantages of the presentinventions have been depicted and described, there are many moreembodiments, applications and advantages possible without deviating fromthe spirit of the inventive concepts described herein. Thus, theinventions are not to be restricted to the preferred embodiments,specification or drawings.

What is claimed is:
 1. A scan processing system for processing x-raytransmissiveness information comprising: a scan processing module atwhich related x-ray transmissiveness information from multiple x-rayilluminations of an object to be imaged by an x-ray source received by adetector comprising a plurality of detector elements, are dynamicallysummed; a random access memory at which intermediate results from saidscan processing module are temporarily stored and communicated back tosaid scan processing module; and a data transmitter, said datatransmitter coupled to said scan processing module for reception ofimage information from said scan processing module, and said datatransmitter having a transmitter output communicatively coupled to animage reconstruction system.
 2. The scan processing system of claim 1wherein said random access memory comprises a plurality of cache memoryunits.
 3. The scan processing system of claim 2 wherein said pluralityof cache memory units comprises a first and a second cache memory unit.4. The scan processing system of claim 3 wherein said intermediateresults are stored by said first cache memory unit, said intermediateresults are added to said related x-ray transmissiveness information bysaid scan processing module to generate summed x-ray transmissivenessinformation, and said summed x-ray transmissiveness information storedat said second cache memory unit.
 5. The scan processing system of claim1 wherein said related x-ray transmissiveness information received atsaid detector corresponds to x-ray transmissiveness informationresulting from more than one x-ray illumination of said object from oneaperture of a plurality of apertures in a collimator grid: saidcollimator grid positioned between said x-ray source and said detector.6. The scan processing system of claim 1 wherein said scan processingmodule comprises one or more adders to sum said related x-raytransmissiveness information.
 7. The scan processing system of claim 1wherein said related x-ray transmissiveness information are summed foralignment of an x-ray imaging system.
 8. The scan processing system ofclaim 1 wherein said related x-ray transmissiveness information aresummed to generate data for use in an image reconstruction system.
 9. Animaging system comprising: an x-ray source comprising a charged particlebeam source and an x-ray target assembly, said charged particle beamsource generating a particle beam that scans across said x-ray targetassembly in a scanning pattern said scanning pattern rescans a portionof said x-ray target assembly a plurality of times during a frame; aplurality of detection elements configured to detect x-rays generated bysaid x-ray target assembly; each of said plurality of detection elementshaving a detection output comprising x-ray transmissiveness information;and a scan processing system comprising a scan processing module, saidscan processing module configured to dynamically combine related x-raytransmissiveness information received at said plurality of detectionelements; and a cache memory, said cache memory configured to storeintermediate results from said scan processing system.
 10. The imagingsystem of claim 9 further comprising an image reconstruction modulecomprising an image reconstruction module input, said imagereconstruction module input communicatively connected to said scanprocessing system.
 11. The imaging system of claim 9 wherein saidrelated x-ray transmissiveness information are summed to generate datafor use in reconstructing an image.
 12. The imaging system of claim 10wherein said cache memory comprises a plurality of cache memory units.13. The imaging system of claim 12 wherein said plurality of cachememory units comprises a first and a second cache memory unit.
 14. Thescan processing system of claim 13 wherein an intermediate result fromsaid scan processing module is stored by said first cache memory unit,said intermediate result is added to said related x-ray transmissivenessinformation by said scan processing module to generate summed x-raytransmissiveness information, and said summed x-ray transmissivenessinformation stored at said second cache memory unit.
 15. The scanprocessing system of claim 9 wherein said related x-ray transmissivenessinformation received at said plurality of detection elements correspondsto x-rays received from one or more apertures of a plurality ofapertures in a collimator grid.
 16. The imaging system of claim 9wherein said scan processing module comprises one or more adders. 17.The imaging system of claim 9 wherein said related x-raytransmissiveness information are summed for alignment of said imagingsystem.